Sequential Synthesis Methodology Yielding Well-Defined Porous 75%SrTiO3/25%NiFe2O4 Nanocomposite

In this research, we reported on the formation of highly porous foam SrTiO3/NiFe2O4 (100−xSTO/xNFO) heterostructure by joint solid-state and sol-gel auto-combustion techniques. The colloidal assembly process is discussed based on the weight ratio x (x = 0, 25, 50, 75, and 100 wt %) of NiFe2O4 in the 100−xSTO/xNFO system. We proposed a mechanism describing the highly porous framework formation involving the self-assembly of SrTiO3 due to the gelation process of the nickel ferrite. We used a series of spectrophotometric techniques, including powder X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), N2 adsorption isotherms method, UV-visible diffuse reflectance spectra (UV-Vis DRS), vibrating sample magnetometer (VSM), and dielectric measurements, to investigate the structural, morphological, optical, magnetic, and dielectric properties of the synthesized samples. As revealed by FE-SEM analysis and textural characteristics, SrTiO3-NiFe2O4 nanocomposite self-assembled into a porous foam with an internally well-defined porous structure. HRTEM characterization certifies the distinctive crystalline phases obtained and reveals that SrTiO3 and NiFe2O4 nanoparticles were closely connected. The specific magnetization, coercivity, and permittivity values are higher in the 75STO/25NFO heterostructure and do not decrease proportionally to the amount of non-magnetic SrTiO3 present in the composition of samples.

by joint solid-state reaction and sol-gel auto-combustion technique. The mechanism of colloidal assembly is discussed in the present study based on the weight ratio x (x = 0, 25,50,75, and 100 wt %) of NiFe 2 O 4 in the 100−x STO/ x NFO system. The rational synthesis presented in this study, with emphasis on the colloidal assembly of the attachment of the NiFe 2 O 4 particles in the presence of SrTiO 3 , represents a step forward in reaching multifunctional properties. These advances are at the core of progress in photocatalytic applications, including water remediation (dyes and pharmaceutical drugs degradation) and especially in water splitting (photocatalytic generation of H 2 ). Thus, synthesized porous 75% SrTiO 3 / 25% NiFe 2 O 4 nanocomposite materials offer flexibility for integrating multiple functionalities such as catalytic activity, adsorption capacity, photocatalytic activity, and magnetic properties that make them attractive for applications in photocatalysis.

Porous Heterostructure Foams 100−x STO/ x NFO Multistep Synthesis Method
Analytical grade precursors have been used to synthesize our materials, such as strontium carbonate (SrCO 3 , 99%, UCB chemicals; Brussels, Belgium), titanium dioxide (TiO 2 , 99%, Loba Feinchemie; Fischamend, Austria), nickel (II) nitrate (Ni (NO 3 ) 2 ·6H 2 O, 97%, Sigma Aldrich; Steinheim am Albuch, Baden-Württemberg, Germany), iron (III) nitrate (Fe (NO 3 ) 3 ·9H 2 O, 98%] Sigma Aldrich; Steinheim am Albuch, Baden-Württemberg, Germany), glycine (NH 2 CH 2 COOH, 99.7%, Merck; Steinheim am Albuch, Baden-Württemberg, Germany). First, the solid-state technique was used for the fabrication of the pristine SrTiO 3 (STO) nanoparticles by mixing strontium carbonate and titanium dioxide with a molar ratio of 1:1. To ensure high homogeneity, the mixture was ground in an agate mortar for 6 h after adding a few drops of ethanol. Thereafter, the powder was calcined in air for 9 h at a temperature of 1000 • C. Pristine NiFe 2 O 4 (NFO) were synthesized via a sol-gel auto-combustion method by mixing nickel nitrate, iron nitrate and glycine with a molar ration of 1:2:1 in 10 mL total volume of distilled water. The solution was agitated at speed of 300 rpm and temperature of 80 • C for 60 min until gelation occurs. Then, the as-prepared gel was heated on a sand bath for 6 h with a temperature-increasing step of 50 • C up to 350 • C, where the auto-ignition started suddenly, quickly, and violently with the flame. The typical experimental procedure used for the synthesis method is given as follows: first, the pristine synthetized STO powder is mixed with nickel nitrate, iron nitrate, and glycine with a molar ration of 1:2:1, in 10 mL of distilled water. Thereafter, gelation was achieved after 30 min under magnetic agitation of the solution at a speed of 300 rpm and temperature of 80 • C, followed by auto-ignition of the as-prepared gel in a sand bath for 6 h with a temperature increasing step of 50 • C up to 350 • C. In the end, the powder was kept for 9 h at a temperature of 1100 • C to obtain a stabilized structure. Thus, the newly heterostructures, designated as of 100−x STO/ x NFO, were obtained at a different mass percentage of NiFe 2 O 4 . Otherwise, to get insight over system design, primordially we consider the structural, magnetic, and optical properties translated in terms of morphology.

Porous Heterostructure 100−x STO/ x NFO Characterization
Phase identification and structural properties of starting nanomaterials calcined at 1000 • C and composite ceramics, obtained at 1100 • C, were determined by X-ray diffraction (Shimadzu LabX 6000; (Tokyo, Japan); diffractometer using Cu-Kα radiation (λ = 1.5406 Å)). The intensity and voltage of the X-ray source were set at 40 mA and 40 kV, respectively. The samples were scanned in reflection mode in the range 20-80 • in 2θ with a step increment of 0.02 • per step and a time per step of 0.2 s. Next, the morphology of the samples was studied using FE-SEM analysis, carried out on a Carl Zeiss NEON 40EsB; (Jena, Germany) with thermal Schottky field emission and accelerated Ga ions column. The FE-SEM micrographs were collected at different acceleration voltage and magnifications (1.8 kV, 50 kX 20 kV, 200 kX; 1.8 kV, 100 kX; and 5 kV, 150 kX). High-resolution electron micrographs of the calcined samples were taken by using a transmission electron microscope equipped with energy-dispersive X-ray spectroscopy (EDS) (Carl Zeiss LIBRA 200 MC UHR-TEM); (Jena, Germany) (at magnification of 700 kX and accelerating voltage HV of 200 kV. The samples for high resolution transmission electron microscopy (HR-TEM) were prepared by ultrasonically dispersing the powder in isopropanol and allowing a drop of this to dry on a carbon-coated copper grid. The specific surface area and the pore size of the calcined samples were measured by gas adsorption-desorption isotherms method, using nitrogen as adsorbate at 78 K on a Quantachrome Nova 2200 analyzer and the samples being degassed for 4 h at 250 • C before testing. UV-Vis absorption spectrum of the 75 STO/ 25 NFO nanocomposite in the wavelength range of 200-1100 nm was investigated by using a Shimadzu UV-Vis spectrophotometer. The energy of the bandgap (Eg) was calculated using Tauc's method [35]. The variation of magnetic properties of the composite ceramics was studied by hysteresis loop obtained using a vibrating sample magnetometer (VSM, Princeton/Lakeshore M3900, Lake Shore Cryotronics, Westerville, OH, USA)), at room temperature under a magnetic field in the range of ±10 kOe. Dielectric properties of the calcined samples, in a wide temperature range (20-200 • C) and for the frequency 2 ÷ 2 × 10 6 Hz, were carried out on the Ag-electrodes applied to the polished faces of the pelletized samples by using an Agilent E4980A Precision LCR Meter (Santa Clara, CA, USA). The cylindrical pellets with a diameter of 12 mm and 1.5 mm thickness were obtained at 140 MPa by using a Hand Press (Carver Inc., Model 4350.L, Wabash, IN, USA) and thermal treatment of 3 h at 700 • C.

Microstructural Characterization
The crystal structure and crystallinity of nanopowders, as pristine SrTiO 3 and NiFe 2 O 4 , as well as 75 STO/ 25 NFO porous-foam nanocomposite, were analyzed using X-ray diffraction analysis performed at room temperature in the 20-80 • (2θ degree) range ( Figure 1).
The diffraction peaks can be indexed to ICSD no. 98-009-1899 for SrTiO 3 and to ICSD no. 98-016-5448 for NiFe 2 O 4 , obtained via a sol-gel auto-combustion method without any other impurity or secondary phases. The pristine SrTiO 3 obtained by solid-state reaction presents a cubic structure and a Sr-rich Ruddlesden-Popper (RP) phase [36,37]. After combining simultaneously, the above two synthetic methodologies, 75 STO/ 25 NFO porous-foam nanocomposite was obtained as presented in Figure 1b. The diffraction peak of both, perovskite and inverse spinel, respectively are tracked together in the powder XRD pattern of the obtained foam. Thus, the sequential synthesis methodology addressed in this study allows the formation of a novel porous foam nanocomposite 75 STO/ 25 NFO. Compared with the cubic perovskite and the inverse spinel-type structures, the diffraction lines of both SrTiO 3 and NiFe 2 O 4 phases are present in the XRD pattern of the nanocomposites ( Figure S1). The traditional crystallographic approach for structure determination (Rietveld refinement) was sufficient to confirm the crystal structure of the nanocomposites ( Figures S2-S7). Rietveld analysis showed that the rutile TiO 2 is present as a secondary phase because SrTiO 3 is locally nonstoichiometric having Sr-rich RP phase. Using the normalized reference intensity ratio method (RIR), the percentage of rutile TiO 2 phase in the 75 STO/ 25 NFO porous-foam nanocomposite was determined to be about 2%.
According to data output from Rietveld refinement, as presented in Table 1, the crystallite size is 45 nm for NiFe 2 O 4 and 49 nm for SrTiO 3 , respectively. In the case of composite, the Rietveld refinement was made by indexing the present phases, and their crystallite sizes were calculated by decomposing each corresponding diffraction peak, calculating FWHM. Therefore, the crystallite size values for NFO and STO were found to be 55 and 50 nm, respectively.
From the FE-SEM analysis of the 75 STO/ 25 NFO porous-foam nanocomposite, the hierarchically porous material was formed by joint solid-state and sol-gel auto-combustion techniques. Hierarchical porosity is quite desirable for adsorption and photocatalytic processes and thus 75 STO/ 25 NFO as visible-light-driven porous-foam nanocomposite with high contact surface area, high storage volume, ready mass transport, and a well-controlled porosity, which grant to this material a very high adsorption capacity and a high photocatalytic efficiency. The diffraction peaks can be indexed to ICSD no. 98-009-1899 for SrTiO3 and to ICSD no. 98-016-5448 for NiFe2O4, obtained via a sol-gel auto-combustion method without any other impurity or secondary phases. The pristine SrTiO3 obtained by solid-state reaction presents a cubic structure and a Sr-rich Ruddlesden-Popper (RP) phase [36,37]. After combining simultaneously, the above two synthetic methodologies, 75STO/25NFO porous-foam nanocomposite was obtained as presented in Figure 1b. The diffraction peak of both, perovskite and inverse spinel, respectively are tracked together in the powder XRD pattern of the obtained foam. Thus, the sequential synthesis methodology addressed in this study allows the formation of a novel porous foam nanocomposite 75STO/25NFO. Compared with the cubic perovskite and the inverse spinel-type structures, the diffraction lines of both SrTiO3 and NiFe2O4 phases are present in the XRD pattern of the nanocomposites ( Figure S1). The traditional crystallographic approach for structure determination (Rietveld refinement) was sufficient to confirm the crystal structure of the nanocomposites ( Figures S2-S7). Rietveld analysis showed that the rutile TiO2 is present as a secondary phase because SrTiO3 is locally nonstoichiometric having Sr-rich RP phase. Using the normalized reference intensity ratio method (RIR), the percentage of rutile TiO2 phase in the 75STO/25NFO porous-foam nanocomposite was determined to be about 2%.  The pore formation mechanism in 75 STO/ 25 NFO is shown in Figure 2. Particle interaction is dependent on sol-gel auto-combustion, which plays a key role in determining the morphology of the porous nanocomposite. Herein, a probable mechanism of pore formation in a highly porous framework is proposed and involves the self-assembly of SrTiO 3 via the gelation process of the nickel nitrate, iron nitrate, and glycine. In the first step, polyhedral SrTiO 3 nanoparticles were synthesized via the solid-state reaction of stron-Nanomaterials 2022, 12, 138 6 of 16 tium carbonate and titanium dioxide with a molar ratio of 1:1 and thermally treated in the air for 9 h at a temperature of 1000 ºC. Next, these polyhedral SrTiO 3 nanoparticles are used as a template to form a 3D hierarchical xerogel composed of nickel and iron glycinate at a temperature of 80 ºC for 60 min. Finally, the highly porous framework was converted to porous-foam 75 STO/ 25 NFO nanocomposite on a sand bath for 6 h with a temperature-increasing step of 50 • C until auto-ignition occurs. To understand the morphological evolution of the 75STO/25NFO nanocomposite into a hierarchical porous-foam structure, composition-dependent experiments were performed. Figure 3 depicts the FE-SEM images of the SrTiO3 template obtained from the solidstate reaction, pure NiFe2O4 via a sol-gel auto-combustion, and its composition-dependent morphological evolution following different weights ratio x = 0, 50, 75, and 100 wt % of NiFe2O4 in the 100-xSTO/xNFO system. Histograms, inserted in Figure 3, show the particle size distributions of the calcined starting materials and the 50STO/50NFO and the 25STO/75NFO. It is clear that with the NiFe2O4 increasing content in system, the average particle size decreases, due to the smaller particle size of the ferrite. To understand the morphological evolution of the 75 STO/ 25 NFO nanocomposite into a hierarchical porous-foam structure, composition-dependent experiments were performed. Figure 3 depicts the FE-SEM images of the SrTiO 3 template obtained from the solidstate reaction, pure NiFe 2 O 4 via a sol-gel auto-combustion, and its composition-dependent morphological evolution following different weights ratio x = 0, 50, 75, and 100 wt % of NiFe 2 O 4 in the 100−x STO/ x NFO system. Histograms, inserted in Figure 3, show the particle size distributions of the calcined starting materials and the 50 STO/ 50 NFO and the Nanomaterials 2022, 11, x FOR PEER REVIEW 8 of 18 Histograms (inserted) showing the particle size distribution of the starting materials and the two composites by using the ImageJ software. The mean value of the particle size and the errors (standard deviation (SD) were calculated by a Gaussian distribution applied to the histograms obtained.
As seen in Figure 3b, the template consists of highly nonuniform polyhedral nanoparticles. When these polyhedral nanoparticles were reacted with different amounts of aqueous Ni 2+ and Fe 3+ solutions (concerning composition stoichiometry) at room temperature in the presence of glycine, they form nonhomogeneous frameworks. The framework becomes more and more disordered as the amount of NiFe2O4 increases, while highly porous-foam nanocomposite is obtained at the 3:1 mass ratio for SrTiO3:NiFe2O4 (Figure 2ad), thereby indicating the optimal quantity of the reactants. Moreover, the selection of glycine, as fuel and chelating agent, in this synthesis was made according to its characteristic combustion temperature, because it affects the size of the crystallites, the structural stabilization, and the morphology. Glycine ignites at low temperatures, but the exothermic reaction is strong and violent with a large amount of gas released and high enthalpy leading to an increase in crystallites along with a good formation and high purity of the spinel-type structure. Our additional experiments reveal that when the concentration of the aqueous Ni 2+ and Fe 3+ glycinate was increased, the porous-foam-like structure partially collapsed to accommodate the formation of a nonhomogeneous system mixture of the two components.  25 STO/ 75 NFO [scale 400 nm, electron high tension EHT = 20 KV and MAG = 35 kX] composites. Histograms (inserted) showing the particle size distribution of the starting materials and the two composites by using the ImageJ software. The mean value of the particle size and the errors (standard deviation (SD) were calculated by a Gaussian distribution applied to the histograms obtained.
As seen in Figure 3b, the template consists of highly nonuniform polyhedral nanoparticles. When these polyhedral nanoparticles were reacted with different amounts of aqueous Ni 2+ and Fe 3+ solutions (concerning composition stoichiometry) at room temperature in the presence of glycine, they form nonhomogeneous frameworks. The framework becomes more and more disordered as the amount of NiFe 2 O 4 increases, while highly porous-foam nanocomposite is obtained at the 3:1 mass ratio for SrTiO 3 :NiFe 2 O 4 (Figure 2a-d), thereby indicating the optimal quantity of the reactants. Moreover, the selection of glycine, as fuel and chelating agent, in this synthesis was made according to its characteristic combustion temperature, because it affects the size of the crystallites, the structural stabilization, and the morphology. Glycine ignites at low temperatures, but the exothermic reaction is strong and violent with a large amount of gas released and high enthalpy leading to an increase in crystallites along with a good formation and high purity of the spinel-type structure. Our additional experiments reveal that when the concentration of the aqueous Ni 2+ and Fe 3+ glycinate was increased, the porous-foam-like structure partially collapsed to accommodate the formation of a nonhomogeneous system mixture of the two components.
To observe in more detail the morphology nature and lattice structure of 75 STO/ 25 NFO, both transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) imaging were performed. Figure 4a-d reveals the interconnected NiFe 2 O 4 nanoparticles with polyhedral SrTiO 3 nanoparticles leading to porous nature of 75 STO/ 25 NFO nanocomposite, as well as another interesting feature of small NiFe 2 O 4 particle attachments indicated by the yellow arrows present on the surface of the SrTiO 3 . Furthermore, it can be observed from Figure 4d that the optimum 3:1 ratio of components forms bulk chains rather than a core-shell type structure. The TEM energy dispersive spectroscopy (TEM-EDS) elemental mapping images of 75 STO/ 25 NFO nanocomposite reveals the uniform distribution of Sr, Ti, and O atoms in the nanocomposite ( Figure S8) while Fe and Ni atoms appear attached and among to the surface and interconnected respectively to the perovskite phase. To observe in more detail the morphology nature and lattice structure of 75STO/25NFO, both transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) imaging were performed. Figure 4a-d reveals the interconnected NiFe2O4 nanoparticles with polyhedral SrTiO3 nanoparticles leading to porous nature of 75STO/25NFO nanocomposite, as well as another interesting feature of small NiFe2O4 particle attachments indicated by the yellow arrows present on the surface of the SrTiO3. Furthermore, it can be observed from Figure 4d that the optimum 3:1 ratio of components forms bulk chains rather than a core-shell type structure. The TEM energy dispersive spectroscopy (TEM-EDS) elemental mapping images of 75STO/25NFO nanocomposite reveals the uniform distribution of Sr, Ti, and O atoms in the nanocomposite ( Figure S8) while Fe and Ni atoms appear attached and among to the surface and interconnected respectively to the perovskite phase.  Figure 5 shows the nitrogen adsorption-desorption isotherms measurements of the composites and the corresponding average pore size distributions. It is found that all isotherms are of type IV with a H3 narrow hysteresis curve according to the IUPAC classification, characteristic of mesoporous materials (pores size in the range of 2.0-50.0 nm) [38]. The type of hysteresis loop provides information about the shape and connectivity of the inner pores [39], which plays an important role in the rate of gas adsorption. In our materials, H3 type hysteresis loops are associated with a porous structure that has pores with irregular size and shapes [40]. For all analyzed samples a gradual increase of the amount of N2 adsorbed from relatively small values of relative pressure is observed. However, the  Figure 5 shows the nitrogen adsorption-desorption isotherms measurements of the composites and the corresponding average pore size distributions. It is found that all isotherms are of type IV with a H 3 narrow hysteresis curve according to the IUPAC classification, characteristic of mesoporous materials (pores size in the range of 2.0-50.0 nm) [38]. The type of hysteresis loop provides information about the shape and connectivity of the inner pores [39], which plays an important role in the rate of gas adsorption. In our materials, H 3 type hysteresis loops are associated with a porous structure that has pores with irregular size and shapes [40]. For all analyzed samples a gradual increase of the amount of N 2 adsorbed from relatively small values of relative pressure is observed. However, the sudden increase in the amount of N 2 adsorbed at low values of relative pressure, P/P 0 , proves that the samples present relatively large specific surface areas. Moreover, hys-teresis loop at high relative pressures can be found, indicating the presence of porous structure, especially for the 75 STO/ 25 NFO nanocomposite. The pore size distribution curve of 75 STO/ 25 NFO shows a monomodal distribution with average pore diameters of between 13.97 and 15.86 nm (calculated from BJH desorption and adsorption isotherms, respectively) [41]. The 50 STO/ 50 NFO exhibits a linear pore distribution with average pore diameter between 8.23 nm and 7.44 nm and the 25 STO/ 75 NFO composite shows a narrow pore-size distribution with pore size of about 7.5 nm. Moreover, t-Plot micropore volume [42] is negative for all composites, meaning the samples do not contain micropores. The calculated textural characteristics of the samples, by using BET, BJH and t-plots methods, are summarized in Table 2. As expected, 75 STO/ 25 NFO nanocomposite has the highest S BET and Langmuir surface area, of 52 and 75 m 2 g −1 , respectively, as well as pore volume, suggesting that meso-pores make the largest contribution to the surface area. In this way, meso-porosity is produced in 75 STO/ 25 NFO composite leading to an ordered porous structure, as confirmed by FE-SEM and HR-TEM. 50 STO/ 50 NFO and 25 STO/ 75 NFO composites showed smaller S BET and pore volume, suggesting the entry of the nitrogen gas molecules was partially restricted [43]. Nearly double values of pore volume and diameter (0.148 nm and 15.86 nm, respectively) and larger size of specific surface area suggest that 75 STO/ 25 NFO nanocomposite can provide more active sites and adsorb more reactive species in further photocatalytic experiments. sudden increase in the amount of N2 adsorbed at low values of relative pressure, P/P0, proves that the samples present relatively large specific surface areas. Moreover, hysteresis loop at high relative pressures can be found, indicating the presence of porous structure, especially for the 75STO/25NFO nanocomposite. The pore size distribution curve of 75STO/25NFO shows a monomodal distribution with average pore diameters of between 13.97 and 15.86 nm (calculated from BJH desorption and adsorption isotherms, respectively) [41]. The 50STO/50NFO exhibits a linear pore distribution with average pore diameter between 8.23 nm and 7.44 nm and the 25STO/75NFO composite shows a narrow poresize distribution with pore size of about 7.5 nm. Moreover, t-Plot micropore volume [42] is negative for all composites, meaning the samples do not contain micropores. The calculated textural characteristics of the samples, by using BET, BJH and t-plots methods, are summarized in Table 2. As expected, 75STO/25NFO nanocomposite has the highest SBET and Langmuir surface area, of 52 and 75 m 2 g −1 , respectively, as well as pore volume, suggesting that meso-pores make the largest contribution to the surface area. In this way, mesoporosity is produced in 75STO/25NFO composite leading to an ordered porous structure, as confirmed by FE-SEM and HR-TEM. 50STO/50NFO and 25STO/75NFO composites showed smaller SBET and pore volume, suggesting the entry of the nitrogen gas molecules was partially restricted [43]. Nearly double values of pore volume and diameter (0.148 nm and 15.86 nm, respectively) and larger size of specific surface area suggest that 75STO/25NFO nanocomposite can provide more active sites and adsorb more reactive species in further photocatalytic experiments.   Figure 6 shows the UV-Vis absorbance spectra of the 75STO/25NFO nanocomposite as a hierarchical porous-foam structure to reveal the optical properties. It is very well studied in the literature that the absorption band of pure SrTiO3 sharply drops at about 388 nm whereas that of the pure NiFe2O4 continuously extends to the visible-light region (300 nm < λ < 1000 nm). This means that SrTiO3 only has a response to the UV light but the pure NiFe2O4 can respond to both the UV and visible light. Using Tauc's relation: ((αhν) 2 = A(hν-Eg)), the bandgap of 1.5 eV is determined by extrapolating the linear portion of the   Figure 6 shows the UV-Vis absorbance spectra of the 75 STO/ 25 NFO nanocomposite as a hierarchical porous-foam structure to reveal the optical properties. It is very well studied in the literature that the absorption band of pure SrTiO 3 sharply drops at about 388 nm whereas that of the pure NiFe 2 O 4 continuously extends to the visible-light region (300 nm < λ < 1000 nm). This means that SrTiO 3 only has a response to the UV light but the pure NiFe 2 O 4 can respond to both the UV and visible light. Using Tauc's relation: ((αhν) 2 = A(hν-Eg)), the bandgap of 1.5 eV is determined by extrapolating the linear portion of the curve to the energy axis as shown in Figure 6 (inset). We can note that the 75 STO/ 25 NFO nanocomposite with a hierarchical porous-foam structure has extended absorption spectra into the visible light region. Interestingly, it is noteworthy that the characteristic absorption peak of NiFe 2 O 4 at about 750 nm still excites in that of synthesized hierarchical porous-foam and is certified further to the earlier reports about the similar composite nanoparticles and from this point of view, the heterojunctions of SrTiO 3 /NiFe 2 O 4 bring up the complementarity that should exist between the two components ( Table 3). Note that the study of these nanomaterials is a complex one, and here is presented only the first stage, in which we wanted to highlight an extremely interesting phenomenon of colloidal attachment of two reference nanomaterials in current technological and environmental applications.
Nanomaterials 2022, 11, x FOR PEER REVIEW 11 of 18 curve to the energy axis as shown in Figure 6 (inset). We can note that the 75STO/25NFO nanocomposite with a hierarchical porous-foam structure has extended absorption spectra into the visible light region. Interestingly, it is noteworthy that the characteristic absorption peak of NiFe2O4 at about 750 nm still excites in that of synthesized hierarchical porous-foam and is certified further to the earlier reports about the similar composite nanoparticles and from this point of view, the heterojunctions of SrTiO3/NiFe2O4 bring up the complementarity that should exist between the two components ( Table 3). Note that the study of these nanomaterials is a complex one, and here is presented only the first stage, in which we wanted to highlight an extremely interesting phenomenon of colloidal attachment of two reference nanomaterials in current technological and environmental applications.    Room temperature hysteresis loops of our pure and composite materials are represented in Figure 7. Indeed, as shown in Figure 7d the recorded room temperature hysteresis loops reveal that the pure SrTiO 3 has no magnetism; however, pristine NiFe 2 O 4 nanopar-ticles show a ferromagnetic behavior with the saturation magnetization (M S ) of about 45 emu g −1 (Figure 7c).
Room temperature hysteresis loops of our pure and composite materials are represented in Figure 7. Indeed, as shown in Figure 7d the recorded room temperature hysteresis loops reveal that the pure SrTiO3 has no magnetism; however, pristine NiFe2O4 nanoparticles show a ferromagnetic behavior with the saturation magnetization (MS) of about 45 emu g −1 (Figure 7c). Considering the presence of the non-magnetic SrTiO3, the values for the synthesized composites with different ratios of NiFe2O4 from 25% up to 75% (Figure 7b), are lower than that of the pure spinel phase and they are between 25 and 39 emu g −1 (see Table  4). It is interesting to note that the magnetization does not decrease proportionally to the amount of non-magnetic SrTiO3 present in the composition of samples (Figure 7b). Moreover, the highest coercivity value is determined for the 75STO/25NFO sample, maybe due to the high magneto crystalline anisotropy, emphasizing the same trend described videsupra. The mentioned phenomenon needs more experimental work corroborated with theoretical simulation to unequivocally assess the magnetic properties of the as-prepared nanocomposites in hierarchical assembly as well in nonhomogeneous form. Considering the presence of the non-magnetic SrTiO 3 , the M S values for the synthesized composites with different ratios of NiFe 2 O 4 from 25% up to 75% (Figure 7b), are lower than that of the pure spinel phase and they are between 25 and 39 emu g −1 (see Table 4). It is interesting to note that the magnetization does not decrease proportionally to the amount of non-magnetic SrTiO 3 present in the composition of samples (Figure 7b). Moreover, the highest coercivity value is determined for the 75 STO/ 25 NFO sample, maybe due to the high magneto crystalline anisotropy, emphasizing the same trend described vide-supra. The mentioned phenomenon needs more experimental work corroborated with theoretical simulation to unequivocally assess the magnetic properties of the as-prepared nanocomposites in hierarchical assembly as well in nonhomogeneous form.

Comparative Analysis of the Dielectric Properties
The evolution with the frequency of the real and imaginary parts of permittivity for the starting material and the composites prepared is comparatively presented in Figure 8a-h. For a better understanding of the extrinsic permittivity of the composite materials, SrTiO 3 was prepared under the same conditions as the composites for the dielectric analysis. The frequency dependence of real and imaginary part of permittivity of pure pristine SrTiO 3 ceramic at few temperatures are shown in Figure 8a,e, showing a different behavior compared to that of composites. At low frequency, the sample presents a strong decay and at high frequency (10 5 -10 6 ) Hz, where all the extrinsic phenomena were canceled, the permittivity tends to its intrinsic values~1100.
On the other hand, the real part of permittivity vs. frequency indicates rather similar dielectric behaviors for all composites. The frequency dispersion is higher with increasing temperature and this increase is enhanced at lower frequencies. The 75 STO/ 25 NFO composite shows higher permittivity at low frequencies, increasing with temperature.
For texample, at f = 1 kHz, the permittivity assumes values of about 457 at 25 • C and 1072 at 120 • C for the 75 STO/ 25 NFO, while for the 25 STO/ 75 NFO composite, of about 131 and 710 at the same temperatures, respectively. Imaginary parts of permittivity (Figure 8f-h) show a strong decrease with frequency from 2 × 10 4 to~2 for 75 STO/ 25 NFO and 3 × 10 4 to 10 for 25 STO/ 75 NFO composites, respectively, at the frequency between 20 Hz to 10 6 Hz and 120 • C. Both composites show a low-frequency decay of the real and imaginary part of permittivity, which strongly increases with temperature. However, apart from the linear variation observed in the case of 50 STO/ 50 NFO composite, the two materials appear to be temperature independent with increasing frequency and temperature. While for 50 STO/ 50 NFO composite, these extrinsic mechanisms seem to be canceled for frequency higher than 10 5 Hz, in case of the 75 STO/ 25 NFO and 25 STO/ 75 NFO composites, they continue to exist in all investigated frequency range. The observed differences show that extrinsic contributions are slightly different in these types of composites. The extrinsic contributions are due to Maxwell-Wagner relaxations and dc-conductivity caused by the slow-charged species activated at low frequencies and high temperatures [46]. The Maxwell-Wagner relaxation is related to inhomogeneities and interfaces [47], and it seems that the better dielectric characteristics obtained for slightly temperature-dependent 75 STO/ 25 NFO composite are a consequence of different microstructural properties, e.g., hierarchical porous-foam morphology by particle attachment. At high frequency (around 10 5 Hz), where the charge defect-associated relaxations are no longer active, the permittivity tends to its intrinsic value [48,49]. For this frequency, at 120 • C, values around~320 for 75 STO/ 25 NFO (independent with temperature),~250 for 75 STO/ 25 NFO (temperaturedependent) and~190 for 25   On the other hand, the real part of permittivity vs. frequency indicates rather similar dielectric behaviors for all composites. The frequency dispersion is higher with increasing temperature and this increase is enhanced at lower frequencies. The 75STO/25NFO composite shows higher permittivity at low frequencies, increasing with temperature. For example, at f = 1 kHz, the permittivity assumes values of about 457 at 25 °C and 1072 at 120 °C for the 75STO/25NFO, while for the 25STO/75NFO composite, of about 131 and 710 at the same temperatures, respectively. Imaginary parts of permittivity (Figure 8f-h) show a strong decrease with frequency from 2 × 10 4 to ~2 for 75STO/25NFO and 3 × 10 4 to 10 for 25STO/75NFO composites, respectively, at the frequency between 20 Hz to 10 6 Hz and 120 °C. Both composites show a low-frequency decay of the real and imaginary part of permittivity, which strongly increases with temperature. However, apart from the linear variation observed in the case of 50STO/50NFO composite, the two materials appear to be temperature independent with increasing frequency and temperature. While for 50STO/50NFO composite, these extrinsic mechanisms seem to be canceled for frequency higher than 10 5 Hz, in case of the 75STO/25NFO and 25STO/75NFO composites, they continue to exist in all investigated frequency range. The observed differences show that extrinsic contributions The temperature dependence of permittivity shows some differences for the composites according to the composition (Figure 8i-k). A broad permittivity maximum can be detected in the range of about 120-140 • C for all composites and it originates from a relaxor character or a reduction of ferroelectric character [50,51]. The shape and the position of the permittivity maximum are affected by an extrinsic dielectric effect that tends to cover the intrinsic ferroelectric behavior in the composites and the permittivity decreases with temperature and frequency. The shape of the permittivity maximum might be explained by local compositional variations and morphological differences [52].

Conclusions
Sequential synthesis methodology addressed in this study enables the successful fabrication of 75 STO/ 25 NFO as porous-foam in nature by joint solid-state and sol-gel auto-combustion technique. The diffraction peak of both, perovskite and inverse spinel, respectively, were tracked together in the powder XRD pattern of the obtained composite. The 75 STO/ 25 NFO composite exhibits a powerful visible light response with a band gap energy of 1.505 eV and a value of the room temperature permittivity of 457. In addition, 75 STO/ 25 NFO composite shows higher permittivity at low frequencies, which is proportional to the temperature increase. Morphological analyses performed by FE-SEM and HRTEM reveal that the consolidation of particles occurs by exact stacking of NiFe 2 O 4 particles along crystal facets of SrTiO 3 . 75 STO/ 25 NFO nanocomposite showed the highest S BET and Langmuir surface area values, of 52 and 75 m 2 g −1 , respectively, as well as pore volume, which can suggest that meso-pores make the largest contribution to the surface area. The values of the specific magnetization increased from 25.4 to 45.1 emu g −1 when the ratio x in 100−x STO/ x NFO increases from 25 to 100. The coercivity showed the highest value of 214 Oe for the 75 STO/ 25 NFO sample. The nanocomposite can be considered as the famous heterojunction connection by sequential synthesis methodology addressed in this study with emphasis on the colloidal assembly of the attachment of the NiFe 2 O 4 particles in the presence of SrTiO 3 .

Supplementary Materials:
The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/nano12010138/s1, Figure S1. The superimposed XRD patterns of perovskite phases (SrTiO 3 ) synthesized with different ratios of spinel phase (NiFe 2 O 4 ); Figure S2. The Rietveld refinement of the 75 STO/ 25 NFO. Black solid line is the best fits to the experimental data, the blue line represents the differences between the experimental and calculated data while the vertical markers represent the Bragg peaks positions in blue for Cubic perovskite phase, in red for Cubic spinel phase and in green for Tetragonal TiO 2 phase; Figure S3. Le Bail refinement from laboratory XRD patterns at RT The Rietveld refinement of SrTiO 3 nanoparticles. Black solid line is the best fits to the experimental data, the blue line represents the differences between the experimental and calculated data while the vertical markers represent the Bragg peaks positions in blue for Cubic perovskite phase and in red for Ruddlesden-Popper phase; Figure S4. The Rietveld refinement of SrTiO 3 with 25% of NiFe 2 O 4 . Black solid line is the best fits to the experimental data, the blue line represents the differences between the experimental and calculated data while the vertical markers represent the Bragg peaks positions in blue for Cubic perovskite phase and in red for Cubic spinel phase and in green for Tetragonal TiO 2 phase; Figure S5. The Rietveld refinement of SrTiO 3 with 50% of NiFe 2 O 4 . Black solid line is the best fits to the experimental data, the blue line represents the differences between the experimental and calculated data while the vertical markers represent the Bragg peaks positions in blue for Cubic perovskite phase, in green for Tetragonal perovskite phase and in red Cubic spinel phase; Figure S6. The Rietveld refinement of SrTiO 3 with 75% of NiFe 2 O 4 . Black solid line is the best fits to the experimental data, the blue line represents the differences between the experimental and calculated data while the vertical markers represent the Bragg peaks positions in blue for Cubic perovskite phase, in green for Tetragonal perovskite phase and in red Cubic spinel phase; Figure S7. The Rietveld refinement of NiFe 2 O 4 nanoparticles. Black solid line is the best fits to the experimental data, the blue line represents the differences between the experimental and calculated data while the vertical markers represent the Bragg peaks positions in blue for Cubic spinel phase; Figure S8. TEM dark field image of NiFe 2 O 4 particle attachment on SrTiO 3 polyhedral nanoparticles (a) and the corresponding elemental mapping for all elements Fe, Ni, Sr, Ti, O (b) Sr, Ti (c) and Fe, Ni (d).

Data Availability Statement:
The data presented in this study are available from the corresponding author on request.